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I am not clear on the precise location. The term "under" is not acceptable to define a location on the foot, as it may have many meanings and is determined by the position of the foot. Instead, cite location as, dorsal, plantar, superior to, inferior to, proximal to, or distal to. Also, indicate the size of the callous (not just large), its characteristics and appearance.

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Without an examination, one could not be certain, but the most common sort of callous in the area you seem to describe is known as a pinch callous,which is caused by rolling off during the push-off phase of gait, especially in the abducted foot and especially were there are medial prominence of the bony structures of the IP joint. Sometimes partial bone resection can help. Often periodic professional reduction of the keratosis is adequate, possibly along with padding to retard its recurrence. Your first step should be a trip to the podiatrist for an actual rather than guessed-at diagnosis.